The prior art has provided process heaters to heat fluids. A common example of a process heater is a boiler that is used to either raise steam from feed water or to superheat steam that has already been generated. Typically, process heaters combust a fuel in the presence of an oxidant, for example, air, to raise the heat necessary to heat the process fluid. In recent years, it has been suggested to incorporate oxygen transport membranes in process heaters in order to produce permeated oxygen to support the combustion in place of air.
The major advantage of using an oxygen transport membrane in a process heater to supply oxygen for the combustion is that steam present within the flue gases resulting from the combustion can be condensed at a higher temperature than in flue gases produced by combustion supported by air alone. The reason for this is that when the combustion is supported by oxygen produced by the oxygen transport membrane, the flue gases essentially contain carbon dioxide and water. When air is used as the combustion oxidant, the flue gases also contain substantial amounts of nitrogen and the water contained in such flue gases will condense at a much lower temperature, typically about 25° C. lower than the case in which the combustion is supported by oxygen. The condensation of the steam at high temperature allows heat that would be otherwise lost in the stack gases to be recovered and recycled for use in preheating the feed to the process heater. As such, a process heater utilizing an oxygen transport membrane can be more thermally efficient than one using air. In addition to the foregoing, since the flue gases essentially contain water and carbon dioxide, the carbon dioxide can be easily sequestered through conventional removal of the water. Moreover, since only a small amount, if any, of nitrogen is present during the combustion, very little NOx is produced from the combustion.
As well known in the art, oxygen transport membranes can be fabricated from ceramics that are formed into plate or tubular elements that when heated to an operational temperature of between about 400° C. and about 1000° C. exhibit oxygen ion transport. When an oxygen containing gas, for instance, air, is contacted on one side of the membrane, known as the cathode side, the oxygen ionizes by gaining electrons. The resultant oxygen ions are transported through the membrane and emerge from an opposite side, known as the anode side, where the oxygen ions combine to form elemental oxygen and in so doing produce electrons. The electrons are transported back from the anode side to the cathode side to ionize the oxygen. If the ceramic material is a mixed conductor, typically a perovskite, the electrons will be transported in the ceramic material itself. Other types of materials use dual phases of an ionic material, such as ceria or yttria stabilized zirconia, that is capable of only transporting the oxygen ions and an electronically conductive phase. The electronically conducting phase is utilized to conduct the electrons. The transport of the oxygen ions is driven by an oxygen partial pressure differential between the cathode and anode sides of the membrane. This partial pressure difference can be created in whole or in part by consuming the oxygen at the anode side through combustion of a fuel.
There have been a variety of designs for process heaters incorporating oxygen transport membranes proposed in the prior art. Such example can be found in U.S. Pat. No. 6,394,043 that incorporates oxygen transport membranes within a combustion chamber to provide oxygen to support combustion of the fuel and thereby to generate heat. Part of the heat generated is used to heat the oxygen transport membrane to its operational temperature. The remaining portion of the heat is used to raise steam or to superheat steam passing through transfer passages extending through the combustion chamber. Flue gases produced from the combustion can be recirculated and mixed with the fuel. In another example, U.S. Pat. No. 6,562,104, a fuel is combusted within a combustion chamber and the heated flue gases are passed in a cross-flow relationship to oxygen transport membranes that are used to generate oxygen. In one embodiment, the oxygen transport membranes and steam tubes are interspersed within a combustion chamber. In another embodiment, the oxygen transport membranes and steam tubes are separated. A combustion chamber contains the steam tubes and the resultant flue gas is passed to the oxygen transport membranes as a sweep gas. The flue gases become enriched in oxygen and are then recirculated to the combustion chamber.
The problem of an oxy-fuel combustion system that utilizes oxygen transport membranes is that the oxygen flux in fuel rich conditions is substantially greater than in fuel lean conditions by an order of magnitude. Thus, in order to have complete combustion in such a system, a large membrane surface area is required to contribute the oxygen necessary for stoichiometric combustion. As will be discussed, the present invention provides a method of heating a fluid by a process heater having integrated oxygen transport membranes that overcomes this problem by not utilizing oxygen transport membranes as the sole source of the oxygen that is used to support the combustion.